1. Field of the Invention
The present invention relates to a transfer image forming apparatus using an electrophotographic process or electrostatic recording process and, more particularly, to an apparatus for forming an image by using a magnetic brush contact charging apparatus as a charging processing means.
2. Related Background Art
As a photosensitive body used in a transfer electrophotographic image forming apparatus, an organic photosensitive body, amorphous-silicon-based photosensitive body (to be referred to as an a-Si-based photosensitive body hereinafter), or the like is often used. The a-Si-based photosensitive body has high surface hardness, exhibits high sensitivity with respect to a semiconductor laser or the like, and suffers almost no deterioration due to repetitive use, and hence is used as an electrophotographic photosensitive body for a high-speed copying machine or laser beam printer (LBP).
As a charging processing means for a photosensitive body, a corona charging apparatus using electric discharge has been put into practice. However, since the a-Si-based photosensitive body has a relative dielectric constant as large as 11 to 12, which is larger than that of an organic photosensitive body, the capacitance is large. As a consequence, for example, the charging ability tends to deteriorate, and image deletion tends to occur due to latent image deletion caused by discharge.
In contrast to this, if the a-Si-based photosensitive body is charged by a contact charging member using a conductive roller, a fur brush, a magnet roller bearing magnetic particles, or the like, since the surface of the a-Si-based photosensitive body is formed by a layer of 109 to 1014 xcexa9cm, a charge potential almost equivalent to the DC component of a bias applied to the contact charging member can be obtained on the photosensitive body surface.
Such a charging method is called xe2x80x9cinjection chargingxe2x80x9d because the photosensitive body is charged by directly injecting charge into the body without using discharge. If this injection charging is used, completely ozoneless, low-power-consumption charging can be performed because the photosensitive body is charged without using any discharge phenomenon as in a case where a corona charging apparatus is used. This technique therefore has attracted a great deal of attention. In addition, this can prevent a deterioration in charging ability and image deletion. Furthermore, since the photosensitive body is charged at a potential near the applied voltage, potential control is facilitated.
In a contact charging apparatus of a magnetic brush scheme as a kind of injection charging scheme, conductive magnetic particles (carriers) are magnetically constrained as a magnetic brush on a magnet directly or on a sleeve incorporating a magnet, and the magnetic brush is brought into contact with the photosensitive body while being stopped or rotated. By applying a voltage to the magnetic brush, charging is started.
FIG. 2 is a cross-sectional view showing an example of the schematic arrangement of a magnetic brush contact charging apparatus 30. The magnetic brush contact charging apparatus of this example is of a sleeve rotation type.
A rotation drum type electrophotographic photosensitive body 1 as a body to be charged is rotated/driven in the clockwise direction indicated by the arrow at a predetermined peripheral velocity (process speed), e.g., 150 mm/sec.
The charging apparatus 30 includes a housing 305 and a nonmagnetic sleeve (charging sleeve) 303 serving as a magnetic brush bearing member which is rotatably mounted in the housing 305 with its lower surface being exposed to the outside.
A magnet roller (permanent magnet roller) 302 serves as a magnetic field generating member, which is inserted in the nonmagnetic sleeve. This magnet roller 302 is a non-rotating fixed member supported on a fixed central shaft 306. The two ends of the nonmagnetic sleeve 303 are rotatably and axially supported on the two end sides of the fixed central shaft 306. The nonmagnetic sleeve 303 is coaxially rotated around the fixed magnet roller 302 at a predetermined peripheral velocity, e.g., 150 mm/sec, in the clockwise direction indicated by the arrow by a driving system (not shown), i.e., in the counter direction with respect to the photosensitive body 1.
Charging magnetic particles (carriers) are stored in the housing 305. A regulating blade 301 serves as a magnetic particle regulating means placed at the opening portion of the housing 305 at a predetermined distance from the nonmagnetic sleeve 303. The charging magnetic particles 304 in the housing 305 are magnetically constrained and borne as a magnetic brush on the outer surface of the nonmagnetic sleeve 303 by the magnetic field generated by the magnet roller 302 in the sleeve. When the charging magnetic particles 304 are rotated/conveyed upon rotation of the nonmagnetic sleeve 303 and pass through the gap between the nonmagnetic sleeve 303 and the regulating blade 301, the layer thickness is regulated to a predetermined value, and the particles are carried out as a magnetic brush 305a of the housing 305.
The nonmagnetic sleeve 303 is placed to oppose the photosensitive body 1 through a gap smaller than the layer thickness of the magnetic brush 304a having undergone the above layer thickness regulation. Therefore, the magnetic brush 304a whose layer thickness is regulated by the regulating blade 301 and conveyed to the gap portion where the nonmagnetic sleeve 303 opposes the photosensitive body 1 upon rotation of the nonmagnetic sleeve 303 comes into contact with the surface of the photosensitive body 1 with a width, and moves in a direction opposite to the moving direction of the surface of the photosensitive body 1 to slide on the surface of the photosensitive body 1. The nip portion width of a magnetic brush contact nip portion n (charged nip portion) is preferably adjusted to 1 to 10 mm.
The magnetic brush 305a passing through the gap portion where the nonmagnetic sleeve 303 opposes the photosensitive body 1 is conveyed back into the housing 305 upon rotation of the nonmagnetic sleeve 303 to be cyclically used.
When the nonmagnetic sleeve 303 is rotated, and a predetermined charging voltage is applied from a charging bias application power supply S to the nonmagnetic sleeve 303, charge is applied from the charging magnetic particles constituting the magnetic brush 305a onto the photosensitive body 1 in the charging nip portion n, and the rotating photosensitive drum surface is contact-charged to a value near a potential corresponding to the applied charging voltage.
In the image forming apparatus using the above a-Si-based photosensitive body, an optical memory formed in image exposing operation causes a potential difference after the photosensitive body is charged. This appears as an image. As a means for solving this problem, an ante-charge exposing apparatus (pre-exposing step) for uniformly exposing the photosensitive body is generally placed between a cleaner and a charging member. With this arrangement, an optical memory of a preceding image on the photosensitive body is erased.
An optical memory in the image forming apparatus using the above a-Si-based photosensitive body will be described in more detail below.
When the a-Si-based photosensitive body is charged, and image exposure is performed, optical carriers are generated to lower the charging potential, thereby forming an electrostatic latent image. At this time, many dangling bonds (uncombined bonds) existing in the a-Si-based photosensitive body are set at a localized level to trap some optical carriers, retarding their migration or decreasing the recombination rate of optical carriers. In the image forming process, therefore, some optical carriers generated by exposure are released from the localized level at the same time an electric field is applied to the a-Si-based photosensitive body in charging operation in the next step. As a consequence, a surface potential difference is caused on the a-Si-based photosensitive body between an exposed portion and a non-exposed portion to produce an optical memory. This finally appears as a density difference in developing operation.
In general, therefore, uniform exposure is performed in the exposing step to excessively increase latent optical carriers in the a-Si-based photosensitive body so as to make the optical carriers uniform on the entire surface, thereby erasing an optical memory. At this time, an optical memory (ghost) can be erased more effectively by increasing the amount of exposure light emitted from an ante-charge exposing apparatus or bringing the wavelength of pre-exposure light near to the spectral sensitivity peak (about 680 to 700 nm) of the a-Si-based photosensitive body.
When this ante-charge exposing apparatus is used in a system using the above magnetic brush contact charging apparatus, so-called carrier adhesion occurs. That is, charging magnetic particles used in the charging apparatus adhere to the photosensitive body.
The charging magnetic particles adhering to this photosensitive body enter a developing apparatus (developing device) located downstream in the rotating direction of the photosensitive body to interfere with toner image developing. Furthermore, in a transferring portion located downstream, such magnetic particles are transferred onto a transferring material together in image transferring operation, resulting in image defects.
This phenomenon becomes conspicuous especially when an a-Si-based photosensitive body requiring an ante-charge exposing apparatus is combined with a magnetic brush charging apparatus.
When a magnetic brush contact charging apparatus is used, carrier adhesion to a photosensitive body occurs almost in proportion to the potential difference between the voltage applied to a charging member and the charge potential of the photosensitive body. For example, in a charging scheme like an AC charging scheme in which such a potential difference always occurs, carrier adhesion occurs from the entire surface of the charging nip portion of a magnetic brush in the longitudinal direction. In this case, the longitudinal direction is a direction perpendicular to the moving direction of the photosensitive body. In the following description, the longitudinal direction indicates this direction unless otherwise specified. If, therefore, no ante-charge exposing apparatus is used, carrier adhesion can be prevented by using a charging scheme like the injection charging scheme described above in which the voltage applied to the charging member is almost equal to the potential of the photosensitive body.
In the injection charging scheme using a magnetic brush contact charging apparatus, charge is injected into a portion that is in direct contact with a photosensitive body to charge the photosensitive body to a potential almost equal to the voltage applied to the charging member. Therefore, as the number of times the portion comes into contact with the photosensitive body increases, the photosensitive body surface is charged, and its potential approaches the voltage applied to the charging member. For this reason, the density of charging magnetic particles of the magnetic brush is increased and the magnetic brush and photosensitive body are rotated in the counter directions (to move their contact surfaces in opposite directions).
As shown in FIG. 3, however, at the two end portions of the magnetic brush 304a on which the magnet 302 does not exist, the magnetic flux density decreases. In addition, since a pressure escapes to the portions located on the outer sides where no charging magnetic particles exist (arrow A), the toner chain of the magnetic brush 304a tend to become coarse. Therefore, the number of times the magnetic brush 304a comes into contact with the photosensitive body 1 decreases, the charging ability of the magnetic brush contact charging apparatus 30 deteriorates.
In addition, when an ante-charge exposing apparatus is used, exposure light enters the charging nip n in which the toner chain of the magnetic brush 304a is coarse, resulting in a decrease in the surface potential of the charged photosensitive body. As a consequence, the potential difference between each end portion of the magnetic brush and the photosensitive body surface increases, and carrier adhesion tends to occur.
For the above reasons, when an ante-charge exposing apparatus for exposing a photosensitive body to erase an optical memory in the above manner is combined with a magnetic brush contact charging apparatus, a large potential difference occurs between the potential of the photosensitive body and the voltage applied to the magnetic brush at each end portion of the magnetic brush contact charging apparatus in the longitudinal direction. Charging magnetic particles then move onto the photosensitive body to compensate for this potential difference while holding charge, resulting in carrier adhesion.
It is an object of the present invention to provide an image forming apparatus which prevents adhesion of magnetic particles from a charging apparatus to an image bearing body.
It is another object of the present invention to provide an image forming apparatus which prevents an image previously formed on an image bearing body from remaining as a memory when an image is formed on the image bearing body.
It is still another object of the present invention to provide an image forming apparatus which erases an optical memory in forming an image on a photosensitive body.
It is still another object of the present invention to provide an image forming apparatus which prevents the occurrence of an optical memory and adhesion of magnetic particles to a photosensitive body when an amorphous silicon photosensitive body and a magnetic brush charging apparatus are used in combination.